diff --git a/src/eval.c b/src/eval.c
index 0d24f76..2cde642 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1311,7 +1311,8 @@
 
 	    lp->ll_dict = NULL;
 	    lp->ll_list = lp->ll_tv->vval.v_list;
-	    lp->ll_li = check_range_index_one(lp->ll_list, &lp->ll_n1, quiet);
+	    lp->ll_li = check_range_index_one(lp->ll_list, &lp->ll_n1,
+				     (flags & GLV_ASSIGN_WITH_OP) == 0, quiet);
 	    if (lp->ll_li == NULL)
 	    {
 		clear_tv(&var2);
diff --git a/src/evalvars.c b/src/evalvars.c
index 202ada1..dd70427 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1647,13 +1647,15 @@
     {
 	lval_T	lv;
 	char_u	*p;
+	int	lval_flags = (flags & (ASSIGN_NO_DECL | ASSIGN_DECL))
+							     ? GLV_NO_DECL : 0;
+	if (op != NULL && *op != '=')
+	    lval_flags |= GLV_ASSIGN_WITH_OP;
 
 	// ":let var = expr": Set internal variable.
 	// ":let var: type = expr": Set internal variable with type.
 	// ":let {expr} = expr": Idem, name made with curly braces
-	p = get_lval(arg, tv, &lv, FALSE, FALSE,
-		(flags & (ASSIGN_NO_DECL | ASSIGN_DECL))
-					   ? GLV_NO_DECL : 0, FNE_CHECK_START);
+	p = get_lval(arg, tv, &lv, FALSE, FALSE, lval_flags, FNE_CHECK_START);
 	if (p != NULL && lv.ll_name != NULL)
 	{
 	    if (endchars != NULL && vim_strchr(endchars,
diff --git a/src/list.c b/src/list.c
index 666fb5a..50bf3af 100644
--- a/src/list.c
+++ b/src/list.c
@@ -760,18 +760,20 @@
 
 /*
  * Get the list item in "l" with index "n1".  "n1" is adjusted if needed.
- * In Vim9, it is at the end of the list, add an item.
+ * In Vim9, it is at the end of the list, add an item if "can_append" is TRUE.
  * Return NULL if there is no such item.
  */
     listitem_T *
-check_range_index_one(list_T *l, long *n1, int quiet)
+check_range_index_one(list_T *l, long *n1, int can_append, int quiet)
 {
-    listitem_T *li = list_find_index(l, n1);
+    long	orig_n1 = *n1;
+    listitem_T	*li = list_find_index(l, n1);
 
     if (li == NULL)
     {
 	// Vim9: Allow for adding an item at the end.
-	if (in_vim9script() && *n1 == l->lv_len && l->lv_lock == 0)
+	if (can_append && in_vim9script()
+					&& *n1 == l->lv_len && l->lv_lock == 0)
 	{
 	    list_append_number(l, 0);
 	    li = list_find_index(l, n1);
@@ -779,7 +781,7 @@
 	if (li == NULL)
 	{
 	    if (!quiet)
-		semsg(_(e_list_index_out_of_range_nr), *n1);
+		semsg(_(e_list_index_out_of_range_nr), orig_n1);
 	    return NULL;
 	}
     }
diff --git a/src/proto/list.pro b/src/proto/list.pro
index 468775e..3f77f04 100644
--- a/src/proto/list.pro
+++ b/src/proto/list.pro
@@ -30,7 +30,7 @@
 int list_append_number(list_T *l, varnumber_T n);
 int list_insert_tv(list_T *l, typval_T *tv, listitem_T *item);
 void list_insert(list_T *l, listitem_T *ni, listitem_T *item);
-listitem_T *check_range_index_one(list_T *l, long *n1, int quiet);
+listitem_T *check_range_index_one(list_T *l, long *n1, int can_append, int quiet);
 int check_range_index_two(list_T *l, long *n1, listitem_T *li1, long *n2, int quiet);
 int list_assign_range(list_T *dest, list_T *src, long idx1_arg, long idx2, int empty_idx2, char_u *op, char_u *varname);
 void f_flatten(typval_T *argvars, typval_T *rettv);
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 0a25ca4..57d205c 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -288,6 +288,12 @@
     s ..= {a: 2}
   END
   v9.CheckDefAndScriptFailure(lines, ['E1105: Cannot convert dict to string', 'E734: Wrong variable type for .='], 2)
+
+  lines =<< trim END
+      var ls: list<string> = []
+      ls[-1] ..= 'foo'
+  END
+  v9.CheckDefExecAndScriptFailure(lines, 'E684: list index out of range: -1', 2)
 enddef
 
 def Test_assign_register()
diff --git a/src/version.c b/src/version.c
index 95a0f0e..d3be7a8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4662,
+/**/
     4661,
 /**/
     4660,
diff --git a/src/vim.h b/src/vim.h
index bff49be..23771fb 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2632,6 +2632,7 @@
 #define TFN_NO_DECL	0x20	// only used for GLV_NO_DECL
 #define TFN_COMPILING	0x40	// only used for GLV_COMPILING
 #define TFN_NEW_FUNC	0x80	// defining a new function
+#define TFN_ASSIGN_WITH_OP	0x100	// only for GLV_ASSIGN_WITH_OP
 
 // Values for get_lval() flags argument:
 #define GLV_QUIET	TFN_QUIET	// no error messages
@@ -2639,6 +2640,7 @@
 #define GLV_READ_ONLY	TFN_READ_ONLY	// will not change the var
 #define GLV_NO_DECL	TFN_NO_DECL	// assignment without :var or :let
 #define GLV_COMPILING	TFN_COMPILING	// variable may be defined later
+#define GLV_ASSIGN_WITH_OP TFN_ASSIGN_WITH_OP // assignment with operator
 
 #define DO_NOT_FREE_CNT 99999	// refcount for dict or list that should not
 				// be freed.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index a171040..a6789b5 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1988,7 +1988,7 @@
 	else
 	    n2 = (long)tv_get_number_chk(tv_idx2, NULL);
 
-	li1 = check_range_index_one(tv_dest->vval.v_list, &n1, FALSE);
+	li1 = check_range_index_one(tv_dest->vval.v_list, &n1, TRUE, FALSE);
 	if (li1 == NULL)
 	    status = FAIL;
 	else